Complete the following. \[ \begin{array}{l}\text { (a) Find the greatest common factor (GCF) of } 15 \text { and } 12 \text {. } \\ \text { GCF }=\square \\ \text { (b) Use the GCF to factor } 15-12 \text {. } \\ 15-12=\square \times(\square-\square)\end{array} \]

To find the greatest common factor (GCF) of 15 and 12, we start by listing their factors. The factors of 15 are 1, 3, 5, and 15, while the factors of 12 are 1, 2, 3, 4, 6, and 12. The highest factor they share is 3. Therefore, the GCF of 15 and 12 is 3. Using the GCF, we can factor the expression 15 - 12. First, we calculate 15 - 12, which equals 3. We can express 3 as its GCF multiplied by the difference of 15 divided by the GCF and 12 divided by the GCF. So, 15 - 12 = 3 × (5 - 4). Thus, we fill in the blank as follows: \[ \begin{array}{l}\text { (a) GCF } = 3 \\ \text{ (b) } 15 - 12 = 3 \times (5 - 4)\end{array} \]